The operational performance level of ejection seat escape systems has increased considerably over the past three decades. During this time period major strides have also been made in digital electronics, microprocessors and large-scale integrated circuits (LSIC). The invention applies this new technology to the control of ejection seat escape systems. Of particular interest here is the application of microprocessors to ejection seats in order that, among other things, the on-seat post ejection sequencing of recovery events could be more nearly optimized for the airspeed and altitude conditions existing at the time of the ejection.
Without the use of a microprocessor, post ejection sequencing of the recovery parachute is provided by the deployment sequencer covered by U.S. Pat. No. 4,448,374 to Duncan. One shortcoming of this prior art deployment sequencer is its inability to closely control the equivalent airspeed for parachute deployment as a function of altitude as the ejection altitude increases from sea level up to 15,000 feet above sea level. Since the maximum safe opening equivalent airspeed of a parachute decreases with increasing altitude, this deployment sequencer cannot allow full use of the parachute capability at all altitudes from sea level up to 15,000 feet.
During this same period covering the past three decades as the performance capability of ejection seat escape systems was improved, the performance envelope of the aircraft in which they were installed was also greatly extended in both airspeed and altitude so that the flying of missions at supersonic speeds at altitudes of 50,000 feet and higher is now common.
In an ejection at such supersonic airspeeds a normal shock wave will form a few inches forward of the ejectee and the seat which will preclude any on-seat measurement of the actual free stream airspeed and static pressure as long as this shock wave exists. The changes in Mach number and static pressure of the airstream as it passes through a normal shock wave are well known and as a function of the free stream static or ambient pressure, p1, and the free stream Mach number M1, the downstream static or ambient pressure, p2, and the downstream Mach number, M2, are given by the following expressions: ##EQU1## For an aircraft flying at Mach two at the time of ejection, the static pressure immediately downstream of the normal shock wave is 4.5 times the freestream static pressure and the downstream Mach number is 0.289 times the freestream Mach number. For example, an aircraft flying at 38,000 fet MSL (3.00 psi static pressure) with a Mach number equal 2.0 (597.5 KEAS) which is within the state-of-the-art ejection seat theoretical escape capability, immediately downstream of the normal shock wave the static pressure is 13.5 psi (equivalent to 2400 feet MSL) and the Mach number is 0.577 (equivalent to 172.5 KEAS). Such low airspeed and altitude values, if sensed by an on-seat sequencing system, would cause immediate recovery parachute deployment with catastrophic results; i.e., the parachute would be destroyed and the ejectee killed.
Any post ejection sequencer dependent upon seat mounted sensors for measuring free stream total pressure and free stream static pressure can be provided erroneous pressure values as a result of: (1) pitot tube blockage by canopy fragment debris or other debris, (2) failed or erroneous reading pressure transducer, (3) excessive seat angular displacement from the face forward attitude, and (4) normal shock waves which occur in supersonic ejections. Therefore, it is important that erroneous pressure readings be recognized and discarded or corrected for through appropriate means in such post ejection sequencers that are dependent upon seat mounted sensors.
The dynamic pressure acting on a body moving through the air is proportional to the square of the true airspeed times the prevailing air density. The air density in turn is proportional to the prevailing static pressure divided by the prevailing air temperature (absolute). Both the static pressure and air temperature decrease with increasing altitude in the troposphere that extends up to an altitude of about 36,000 feet above MSL on a standard day. Above this altitude in the stratosphere the temperature remains constant but the static pressure continues to decrease with increasing altitude. The deceleration of an ejection seat in a horizontal trajectory, subsequent to ejection from an aircraft and after sustainer rocket burnout, is proportional to the dynamic pressure acting on it divided by its total mass. Therefore, the time to decelerate from a very high airspeed at ejection to an airspeed safe for recovery parachute deployment is a function not only of static pressure and dynamic pressure at the point of ejection but also of the prevailing air density, the effective drag area of the ejected seat, and the total mass of the seat and ejectee. In a dive trajectory a longer time for deceleration will be required as a result of earth gravity acting to accelerate the body and, conversely, in a climbing trajectory at ejection from the aircraft a shorter time will be required. Therefore an optimized post ejection sequencer will provide the shortest appropriate time to parachute deployment for all ejectee weights, for all altitudes at which it is safe to deploy the parachute, for hot or cold temperatures, for all aircraft dive, climb or whatever conditions at the time of ejection of the seat with the ejectee therein, and for all supersonic, transonic, or subsonic ejection airspeeds independent of the aircraft from which the ejection takes place or the local airflow conditions over the cockpit during the escape sequence.
Since the well being of the ejectee depends upon the successful operation of the recovery parachute at or below the maximum safe deployment airspeed of the parachute for the prevailing density of the surrounding air mass, and at or below the maximum altitude safe for the ejectee physiologically, it is essential that the parachute not be prematurely deployed under any possible failure condition and that it shall always be deployed upon reaching the maximum safe deployment airspeed and altitude or at some time shortly thereafter even under multiple failure conditions.
In an ejection at an extreme high altitude of 70,000 feet above MSL the time required to descend to the maximum altitude at which parachute deployment is desired will be from 140 to over 200 seconds. Conservation of the battery supply may be required during this time for descent in order that the microprocessor can successfully deploy the parachute when the desired altitude is reached.
Advantages of the Invention
This invention provides a microprocessor controlled post ejection sequencer which will generate an optimum time delay of the recovery parachute of an ejection seat for all ejection conditions within the envelope of the escape system.
Further, the invention provides failsafe operation in appropriate backup modes for certain multiple failures that could occur in the sequencer and provides immunity from any single failure than can occur in the system. The invention also provides means for conserving battery power during the time required to descend to low altitudes in ejections which have taken place at extreme high altitudes. Yet further, the invention provides maximum immunity of the post ejection sequencing from any external electromagnetic environment.